Puny males punch above their weight to preserve genetic diversity in a declining Atlantic salmon population

Abstract

Many salmonid fish populations have anadromous (i.e. migratory) and nonanadromous individuals co-existing in sympatry. The nonanadromous individuals, frequently males, mature at a much smaller size in freshwater without undergoing marine migrations and often successfully fertilize many eggs laid by anadromous females. Because these small males do not recruit to fisheries, they are often not regarded in high esteem by fishers. In this issue of Molecular Ecology, Johnstone et al. (2013) demonstrate that by substantially contributing to reproduction, such males help maintain genetic diversity in a declining population of Atlantic salmon (Salmo salar). Their results show that estimates of effective population size (Ne), obtained by counting the number of anadromous adults returning from sea and correcting for unequal sex ratios, are lower than estimates generated from genetic markers. Many mechanisms are expected to reduce Ne below the adult census population size (N); the opposite pattern of Ne > N observed by Johnstone et al. (2013) is difficult to explain unless the reproductive effort of nonanadromous males is accounted for. The results have important implications for the conservation of small populations and highlight the challenges of relating Ne to N in organisms with complex life histories.